The invention relates to the suppression of multipath distortion in radio receivers, such as those used for receiving television signals.
Multipath reception conditions give rise to ghosts in NTSC television reception. Multipath signals that arrive at the receiver with enough time displacement from the principal signal as to cause discernible ghosts in a received television image are referred to as xe2x80x9cmacro-ghostsxe2x80x9d. Multipath signals which arrive over a transmission path of lesser length than the strongest or xe2x80x9cprincipalxe2x80x9d signal reach the receiver earlier and are referred to as xe2x80x9cpre-ghostsxe2x80x9d; the ghost images they cause in a received television image appear to the left of the desired image. Pre-ghosts occurring in off-the-air reception can be displaced as much as six microseconds from the xe2x80x9cprincipalxe2x80x9d signal, but pre-ghosts preceding the principal signal by more than four microseconds are rare. Multipath signals which arrive over a transmission path of greater length than the strongest or xe2x80x9cprincipalxe2x80x9d signal reach the receiver later and are referred to as xe2x80x9cpost-ghostsxe2x80x9d; the ghost images they cause in a received TV image appear to the right of the desired image. Typically, the range for post-ghosts extends to forty microseconds displacement from the xe2x80x9cprincipalxe2x80x9d signal, with most post-ghosts occurring in a sub-range that extends to ten microseconds displacement. Multipath signals that arrive at the receiver with too little time displacement from the principal signal as to cause discernible ghosts in a received television image, but which affect transient response, are referred to as xe2x80x9cmicro-ghostsxe2x80x9d. Macro-ghosts are more common in over-the-air terrestrial broadcasts than cablecasting, in which micro-ghosts commonly occur because of reflections introduced by un-terminated or mis-terminated cables.
Ghosts are a problem in digital television (DTV) transmissions as well as in NTSC analog television transmissions, although the ghosts are not seen as such by the viewer of the image televised by DTV. Instead, the ghosts cause errors in the data-slicing procedures used to convert symbol coding to binary code groups. If these errors are too frequent, the error correction capabilities of the DTV receiver are overwhelmed, and there is catastrophic failure in the DTV television image. If such catastrophic failure occurs infrequently, it can be masked to some extent by freezing the last transmitted good DTV image, such masking being less satisfactory if the DTV images contain considerable motion content. The catastrophic failure in DTV image reception may be accompanied by loss of sound, which is harder to conceal than momentary loss of video. Loss or break-up of sound may occur by itself, also.
Filtering to suppress macro-ghosts is often referred to as xe2x80x9cghost-cancellationxe2x80x9d filtering, with filtering to suppress micro-ghosts being referred to as xe2x80x9cchannel equalizationxe2x80x9d. For the sake of brevity, in this specification the term xe2x80x9cequalizerxe2x80x9d will be used generically to describe a filter that suppresses both micro-ghosts and macro-ghosts.
Baseband equalization of demodulated signals can be done with digital filters sampling at the Nyquist or symbol rate of the signal being equalized. Such equalization is called xe2x80x9csynchronous equalizationxe2x80x9d, and equalization cannot be satisfactorily achieved at lower effective sampling rates. If adaptation of the coefficients of the digital filters is to be done by decision-feedback method, synchronous equalization is not satisfactory when multipath distortion is susceptible to change at appreciable rate. Such reception conditions are commonly referred to as xe2x80x9cdynamic multipath conditionsxe2x80x9d, and the multipath distortion occurring under such reception conditions is commonly referred to as xe2x80x9cdynamic multipath distortionxe2x80x9d. The signal to be equalized must be oversampled to obtain the bandwidth in the feedback loop necessary to track changing multipath distortion. Equalization is done by a digital filter or filters having the delay between successive taps a proper fraction of that in the digital filter(s) used for synchronous equalization. Accordingly, baseband equalization of oversampled demodulated signal is termed xe2x80x9cfractional equalizationxe2x80x9d.
Aside from considerations of sampling rate, two basic types of equalization have been employed in the prior art, namely, baseband equalization of demodulated signal and passband equalization for signal modulating a carrier wave. Adaptive digital lowpass filters called xe2x80x9cbaseband equalizersxe2x80x9d are used in baseband equalization. An adaptive digital filter used as a baseband equalizer has weighting coefficients that are adjusted responsive to decision-feedback error signal that is extracted from the demodulated radio signal or to received training signal extracted from selected portions of the demodulated radio signal. Passband equalization as known in the art uses adaptive digital bandpass filters called xe2x80x9cpassband equalizersxe2x80x9d for supplying equalized responses to modulated carrier waves. Typically, the modulated carrier wave is an intermediate-frequency signal derived from a transmitted radio-frequency signal that has been selected for reception. Since passband equalization is performed before demodulation, persons skilled in the art of digital communications radio receiver design have favored its use for radio-frequency signals using modulation resulting in the carrier being central to its sidebands. Examples of such modulation are double-sideband amplitude modulation (DSB AM), quadrature amplitude modulation (QAM), binary phase-shift keying (BPSK) and quadrature phase-shift keying (QPSK). Passband equalization is preferred because the demodulation results, being already equalized, are more reliably suitable for carrier synchronization.
In passband equalization as practiced in the prior art, a digital bandpass filter used as a passband equalizer has its weighting coefficients calculated in accordance with a baseband-to-bandpass transformation of the weighting coefficients that would obtain for an equivalent baseband equalizer. This is the case whether the equalizer has its weighting coefficients determined responsive to training signal extracted from the demodulated radio signal, or has its weighting coefficients adjusted responsive to decision-error feedback signal that is derived from the demodulated radio signal. That is, the signal is demodulated to generate baseband signals that are compared with ideal baseband signals as the basis for determining weighting coefficients for the equivalent baseband equalizer, using a training signal method, a decision-error feedback method or a combination of these two methods.
Passband equalization as known in the art is not particularly well suited for vestigial sideband amplitude-modulation (VSB) signals such as those specified by the 1995 ATSC Digital Television Standard. This is because, with carrier not being midband, baseband-to-bandpass transformation of the equivalent baseband equalizer weighting coefficients results in a passband equalizer having a bandwidth of nearly 12 MHz. If the VSB digitized I-F signal has its carrier in the lower-frequency portions thereof, the carrier must be offset nearly 6 MHz to avoid folding of the bandpass passband. The uppermost frequencies of the VSB digitized I-F signal will be nearly 12 MHz, pushing the sampling rate requirement upward. If the VSB digitized I-F signal has its carrier in the higher-frequency portions thereof, the sampling rate needed to support the 12 MHz bandwidth of the digital filter is still the same. The doubling of sampling rate required by the doubled-bandwidth passband equalizer makes analog-to-digital conversion and phase-splitter filtering considerably more difficult to implement in practice.
Even in passband equalization for signals that employ modulation with the carrier being central to its sidebands, there are previously unrecognized problems associated with using the baseband-to-bandpass transformation of the weighting coefficients of the equivalent baseband equalizer to generate the weighting coefficients for the passband equalizer. The transformation results in passband filtering with amplitude response exhibiting even symmetry about the carrier frequency and phase response exhibiting odd symmetry about the carrier frequency. Reduction of multipath distortion is possible with such passband filtering, since multipath distortion arising from macro-ghosts presents essentially a linear-phase filtering problem. Multipath distortion arising from micro-ghosts (the predominant multipath problem in a cable system) causes asymmetry of the reception channel about the carrier frequency, however. Also, the filtering done to define the passband in a practical receiver design is very apt to introduce asymmetry of the reception channel about the carrier frequency. Using the baseband-to-bandpass transformation of the equivalent baseband equalizer weighting coefficients to generate the weighting coefficients for the passband equalizer does not allow for the upper-sideband frequencies and the lower-sideband frequencies of the modulated carrier to be equalized separately, but forces an equalization on average of upper-sideband and lower-sideband frequencies equidistant from the carrier. Such equalization will permit undesirable distortion in QAM and in multiple-phase shift keying (MPSK) signals, such as QPSK, to remain uncorrected.
In the decision-feedback method disclosed in this specification for adjusting the weighting coefficients of a passband equalizer, the estimates of the actual modulating signal used in multipath-free transmission of the signal that is received are per custom made in the baseband, after demodulation of the received signal that is possibly accompanied by multipath distortion has taken place. In the decision feedback method disclosed in this specification, these estimates are transformed from the baseband to the passband using a modulation process that is the converse of the demodulation process. The results of this re-modulation process are filtered to have the same bandwidth as the received signal, thus to generate a comparison signal against which the actually received passband signal and possible accompanying multipath distortion is differentially compared to generate the decision-feedback error signal. Since in the derivation of the decision-feedback error signal there is no folding of the frequency spectrum of the received signal, which is possibly accompanied by multipath distortion, the equalization of the upper-sideband frequencies and the equalization of the lower-sideband frequencies of the modulated carrier are separable when using the decision-feedback method of the invention.
In the training-signal method disclosed in this specification for adjusting the weighting coefficients of a passband equalizer, a xe2x80x9cpassbandxe2x80x9d training signal with possible multipath distortion is extracted from the actually received passband signal before its demodulation, rather than a xe2x80x9cbasebandxe2x80x9d training signal being extracted from baseband signal following demodulation of the passband signal. The xe2x80x9cpassbandxe2x80x9d training signal is the result of a baseband-to-bandpass transformation of the xe2x80x9cbasebandxe2x80x9d training signal that the prior-art training-signal method employs for adjusting the weighting coefficients of a baseband equalizer. The discrete Fourier transform of the xe2x80x9cpassbandxe2x80x9d training signal with possible multipath distortion as extracted from the actually received passband signal is divided term-by-term by the discrete Fourier transform of the ideal xe2x80x9cpassbandxe2x80x9d training signal, which is free from multipath distortion. The DFT of the ideal xe2x80x9cpassbandxe2x80x9d training signal is stored at the receiver. The DFT resulting from this term-by-term division procedure characterizes the channel through which the transmitted xe2x80x9cpassbandxe2x80x9d training signal is received. The weighting coefficients of the passband equalizer are then calculated as the complex conjugates of the channel DFT terms. The DFTs used in the calculation of the weighting coefficients of the passband equalizer are descriptive of passband signals before there is any frequency spectrum folding associated with the demodulation procedure, so the equalization of the upper-sideband frequencies and the equalization of the lower-sideband frequencies of the modulated carrier are separable when using the training-signal method of the invention.
Radio receivers that down-convert radio-frequency signal to an intermediate-frequency signal offset in its entirety from zero frequency by an offset of no more than a few megacycles are known, being particularly favored for digital television (DTV) signal receivers. The invention in a general aspect thereof is a new type of equalization for such an intermediate-frequency signal. A radio receiver employing this new type of equalization comprises an analog-to-digital converter for digitizing samples of a selected modulated signal as translated to an intermediate-frequency band offset in its entirety from zero frequency, thereby to generate a digitized intermediate-frequency signal; an equalizer including an adaptive digital filter with adjustable filter weights, for providing an equalizer response to the digitized intermediate-frequency signal; a demodulator for demodulating the equalizer response, thereby to provide demodulation results; circuitry for comparing at least a portion of the digitized intermediate-frequency signal with ideal values thereof to generate comparison results; and filter coefficient computation apparatus for determining from those comparison results the adjustable filter coefficients of the adaptive digital filtering required for equalizing the digitized intermediate-frequency signal. That is, the adjustable filter coefficients are calculated based on error signals obtained from modulated I-F signal, rather than on error signals obtained from baseband signals that have been demodulated.
A more specific aspect of the invention concerns calculating by a decision-feedback method the adjustable weights of the adaptive digital filter in the equalizer. To implement such calculation, the radio receiver further comprises circuitry responsive to the demodulation results to generate noise-free estimates of the original modulating signals used for the radio transmission, a modulator for modulating a carrier in accordance with those noise-free estimates of the original modulating signals to generate an estimation of the original radio transmission, and circuitry for generating decision-feedback signal by differentially comparing the adaptive digital filter response with the estimation of the original radio transmission. In a preferred embodiment of this more specific aspect of the invention, the circuitry responsive to the demodulation results to generate noise-free estimates of the original modulating signals used for the radio transmission includes rate-reduction filtering for supplying samples of the demodulation results at symbol rate, quantization circuitry for generating the noise-free estimates of the original modulating signals responsive to the response of said rate-reduction filtering, and re-sampling circuitry for re-sampling the noise-free estimates of the original modulating signals to the same sample rate as the adaptive digital filter response for use as modulating signals by the modulator.
Another more specific aspect of the invention concerns calculating the adjustable weights of the adaptive digital filter by a training signal method. To implement such calculation, the radio receiver further comprises circuitry for extracting training signal of predetermined character and the multipath distortion thereof from the digitized intermediate-frequency signal, and the computer has associated with it a memory for storing the discrete Fourier transform of that training signal of predetermined character without multipath distortion. The training signal of predetermined character without multipath distortion stored in memory is an intermediate-frequency signal that is the baseband-to-passband transform of the baseband training signal that would be used for a baseband equalizer. The computer is arranged for computing the discrete Fourier transform of the training signal of predetermined character and the multipath distortion thereof as extracted from the digitized intermediate-frequency signal. The computer is arranged for then dividing the terms of that discrete Fourier transform by the corresponding terms of the discrete Fourier transform of the training signal of predetermined character without multipath distortion as drawn from the memory therefor, thereby to generate a discrete Fourier transform characterizing the reception channel preceding the adaptive digital filter; The computer is arranged for thereafter calculating the adjustable parameters of the adaptive digital filter so as to compensate against the multipath distortion in the reception channel.